NEW FEATURE: add a formatoption to calculate the rolling mean

[Chilipp]

Summary

add a 'rolling' formatoption to calculate the rolling mean for line plots

Reason

For wiggly lines, it is often not very useful to display them as is but rather to display the rolling mean.

Detailed explanation

Of course one can always do da.rolling(50).mean().psy.plot(...) but since the window (50) might vary, it would be a useful feature to add a formatoption to the psy_simple.plotters.LinePlotter for it. This should then have START priority.